The separation of two nuclear isotopes which differ slightly in mass, such as uranium-235 and uranium-238, generally requires expensive equipment and great amounts of energy. Although gas-centrifugation processes for producing uranium hexafluoride (UF.sub.6) enriched in uranium-235 for nuclear reactor fuels promise to require substantially less energy than gaseous-diffusion processes in present use, the energy requirements for a commercial gas-centrifuge uranium-enrichment facility are nonetheless enormous. Gas centrifuge isotope-enrichment plants are therefore designed to operate as efficiently as possible to minimize the cost of energy consumption. A second significant economic factor is the cost of the plant itself. Gas-centrifuge plants are therefore designed to require as little equipment as practical, consistent with the need for safe operation.
A gas centrifuge for enriching uranium generally has an input into which gaseous uranium hexafluoride is introduced, a light-fraction output out of which a light fraction enriched in .sup.235 UF.sub.6 is withdrawn, and a heavy-fraction output out of which a heavy fraction depleted in .sup.235 UF.sub.6 is withdrawn. One aspect of the performance of such a gas centrifuge is measured by the separation factor .alpha., which is defined by the following formula: ##EQU1## where X.sub.L is the mole fraction of .sup.235 UF.sub.6 in the light fraction and X.sub.H is the mole fractions of .sup.235 UF.sub.6 in the heavy fraction. When the mole fractions X.sub.L and X.sub.H are much less than 1, as is the case where uranium hexafluoride containing 3 mole percent or so of .sup.235 UF.sub.6 for enriched-uranium reactors for electric power generation is being produced, .alpha. approximately equals the ratio of the mole fraction of .sup.235 UF.sub.6 in the light fraction to the mole fraction of .sup.235 UF.sub.6 in the heavy fraction. The value of .alpha. for a particular gas centrifuge depends both on the design of the centrifuge and the conditions under which it is operated.
Typically the separation factors of present-day gas centrifuges under ordinary operating conditions are too low to permit natural-abundance uranium hexafluoride in a single pass through a gas centrifuge to be enriched in uranium-235 sufficiently for use as a fuel in an enriched-uranium reactor. However if a number of gas centrifuges are connected in series so that an output of one centrifuge feeds an input of another, it is possible to enrich uranium hexafluoride progressively to the concentrations of uranium-235 required by such reactors. Thus gas centrifuges for use in producing enriched uranium are ordinarily interconnected to form what are termed cascades. A cascade includes one or more stages of gas centrifuges, the term stage referring to a group of gas centrifuges connected in parallel. Pressure and flow regulators are used to control the flow of uranium hexafluoride among the stages and gas centrifuges. Cascade designs for commercial uranium enrichment plants often incorporate hundreds or even thousands of gas centrifuges. Generally the stages of a cascade are interconnected so that each input of a stage is supplied with uranium hexafluoride from heavy-fraction outputs of stages above it and from light-fraction outputs of stages below it. Thus uranium hexafluoride passing "upward" through a cascade becomes progressively enriched in uranium-235 while uranium hexafluoride passing "downward" becomes progressively depleted in uranium-235. The stages of a cascade are generally divided into two groups, termed enriching stages and stripping stages, an enriching stage being one into which a feedstock is introduced or located higher in the cascade. As used herein, the term cascade can refer to a single stage. The stage of a single-stage cascade would be termed an enriching stage according to the above definition since feedstock is introduced into it.
Since it is costly to enrich the uranium-235 concentration in uranium hexafluoride, mixing two streams of uranium hexafluoride having different degrees of enrichment represents an expense. Cascades are thus ordinarily designed to minimize any such mixing losses in the operation of the cascade. Specific designs of such no-mix cascades for interconnecting various types of gas centrifuges are well known in the art and for conciseness will not be described here. See, for example, H. R. Pratt, Countercurrent Separation Process, Elsevier Publishing Co., New york (1967). It will be noted, however, that considerations concerning the economics of operating a cascade imply that cascades for enriching natural abundance uranium preferably include several stripping stages. Thus, in addition to producing a product fraction enriched in uranium-235 to the degree needed for reactor fuel, cascades of gas centrifuges for enriching natural-abundance uranium hexafluoride also generally produces a waste fraction depleted in .sup.235 UF.sub.6 to a predetermined concentration, typically about 0.02-0.35 mole percent.
A commercial uranium-enrichment facility must provide uranium enriched in uranium-235 to varying degrees since different reactors require fuels having significantly different concentrations of uranium-235. Thus a gas-centrifuge plant would normally include a number of cascades, the cascades having different numbers of enriching stages so that products of different concentrations are produced when the cascades are supplied with natural-abundance uranium hexafluoride.
FIG. 1 depicts a schematic flow graph of a representative prior-art isotope-enrichment unit 100 for enriching natural-abundance uranium hexafluoride. The isotope-enrichment unit 100 will be described in detail to illustrate the flow graph of FIG. 1, since this type of flow graph is employed in connection with various preferred embodiments of the present invention disclosed below. The heavy vertical ines of FIG. 1 represent cascades of gas centrifuges, with filled-in diamonds representing inputs to the cascades and filled-in squares representing outputs. Thus a cascade 112A has an input 114A, a heavy-fraction output 116A, and a light-fraction output 118A. The input 114A of the cascade 112A is connected to a feedstock supply line 140. The heavy-fraction output 116A is connected to a waste-fraction discharge line 142 and the light-fraction output 118A is connected to a first product-fraction discharge line 114. Connected in parallel with the cascade 112A are other operationally-equivalent cascades such as cascades 112J and 112K to form a first subunit 110. An input 114J of the cascade 112J is connected to the feed-stock supply line 140. The heavy-fraction and light-fraction outputs 116J and 118J are respectively connected to the waste-fraction and first product-fraction discharge lines 142 and 144. The cascade 112K and other cascades included in group 110 but not shown in FIG. 1 are connected to the supply line 140 and the discharge lines 142 and 144 in the same manner as the cascades 112A and 112J.
The vertical axis of the flow graph of FIG. 1 represents the concentration of .sup.235 UF.sub.6 relative to the concentration of .sup.235 UF.sub.6 in the feedstock in a logarithmic scale to a base equal to the separation factor .alpha.. The drawings of this application are based on gas centrifuges having a separation factor of 1.5. Although the separation factor of a gas centrifuge typically varies somewhat with its position within a cascade, such variations are ordinarily sufficiently small as to be negligible in the context of understanding the present invention. Referring to the vertical coordinate of the light-fraction outputs 118A-118K and the light-fraction discharge line 144 in FIG. 1, it may be seen that the cascades of the first subunit 110 produce a product fraction enriched in uranium-235 relative to the feedstock ideally by a factor of .alpha..sup.3. For a separation factor .alpha. of 1.5 this corresponds to a .sup.235 UF.sub.6 concentration of about 2.40 mole percent. The cascades of the first subunit 110 also produce a waste fraction depleted in uranium-235 ideally by a factor of .alpha..sup.-3, as may be read from the vertical axis of FIG. 1, which corresponds to a .sup.235 UF.sub.6 concentration of about 0.21 mole percent. One cascade design which is termed in the prior art a one-up/one-down countercurrent cascade can accomplish isotope enrichment and depletion by these factors with six stages of gas centrifuges with separation factors equal to .alpha. in the enriching section and five such stages in the stripping section. Other cascade designs may require different numbers of stages in the two sections.
The isotope enrichment unit 100 also includes a second subunit of cascades 120 and a third subunit 130. Included in the second subunit 120 are cascades 122A-122M which have respectively inputs 124A-124M, heavy-fraction outputs 126A-126M, and light-fraction outputs 128A-128M. The inputs 124A-124M are connected to the feedstock supply line 140 and the waste fraction outputs 126A-126M are connected to the waste fraction discharge line 142. The light fraction outputs 128A-128M are connected to a second product discharge line 146. The cascades of the second group 120 ideally produce a product fraction enriched in uranium-235 by a factor of .alpha..sup.3.multidot.5 and a waste fraction depleted in that isotope by .alpha..sup.-3. Similarly the third subunit 130 produces a product fraction enriched in uranium-235 by a factor of .alpha..sup.4 and a waste fraction having substantially the same concentration as the waste fractions produced by the first and second subunits 110 and 120. The cascades 132A-132N of the third subunit have inputs 134A-134N connected to the feedstock supply line 140, heavy-fraction otuputs 136A-135N are connected to the waste discharge line 142 and the light fraction outputs 138A-138N are connected to a third product discharge line 148.
The cascades of the three subunits have the same number of stripping stages and thus ideally all produce a heavy fraction of the same concentration of uranium-235. Combining the waste fractions produced by the three subunits therefore does not give rise to any significant mixing losses. The three groups of cascades differ, however, in the number of enriching stages. For the one-up/one-down countercurrent cascade design referred to above, cascades of the first, second, and third subunits would all have five stripping stages, but would have respectively six, seven, and eight enriching stages. Thus the isotope-enrichment unit 100 when supplied with natural-abundance uranium hexafluoride on feedstock supply line 140 produces one waste fraction which is withdrawn over the waste discharge line 142 and three product fractions of enriched uranium which are withdrawn over the first, second and third product discharge lines 144, 146, and 148. The rate at which a given product fraction is produced by the isotope-enrichment unit 100 depends on the number and individual capacity of the cascades in the corresponding subunit.
Not only are reactor fuels of different concentrations of uranium-235 required at any given time, but as time goes on the demand profile for fuels enriched to different degrees is likely to change. The changing demand profile for reactor fuels in part results from the fact that nuclear reactors generally require fuels of higher concentrations of uranium-235 for reloading than for starting up. Thus, if, as is likely to be the case, the customers of a new uranium enrichment plant are primarily new nuclear power plants, than the demand profile will shift in time towards higher average concentrations of uranium-235. In addition, reactor designs can be expected to continue to evolve, which leads to changes in the demand profile as new reactors are built.
The changing demand profile for enriched uranium presents a serious problem in designing a uranium-enrichment facility. One way to provide the capability of meeting changes in the demand profile is to have the facility produce a wide spectrum of product fractions of different concentrations of uranium-235 by having numerous subunits of cascades with varying numbers of enriching stages. Uranium hexafluoride of particular concentrations could be made by blending product fractions and changes in the demand profile could be met by adjusting the product blends. However, as noted earlier, such blending is wasteful since isotope separation is such an expensive process. Moreover the average concentration of uranium-235 in the enriched-uranium products of such a plant can only be decreased by blending, and then only by mixing one or more product fractions with natural-abundance uranium or the waste fraction, which in either case results in extremely high mixing losses.
A second way to change the concentrations of products of a uranium enrichment plant which can reduce the problems of mixing losses inherent in blending is to redistribute cascades among the various subunits of the plant by "repiping" some of the cascades to change the number of stages in their enriching sections. To increase the average concentration of .sup.235 UF.sub.6 in the enriched product, for example, a number of the cascades from a subunit having few enriching stages could be repiped into cascades having more enriching stages. Changing even by only one the number of enriching stages for most cascade designs, however, requires changing the number of gas centrifuges in each stage and altering the flow rates of uranium hexafluoride between all of the stages. Thus repiping a large cascade is a major undertaking which involves extended down time and considerable expense. The difficulties which attend repiping a cascade are compounded when the cascade has been handling radioactive material. Moreover, if a commercial gas centrifuge plant is to meet by redistributing cascades among subunits a shift in the product demand profile which stems from supplying a group of reactors of which at one time only 40% require fuel for reloading, but of which 90% require fuel for reloading three years later, then about 27% of the cascades in the plant must be repiped. Repiping over a quarter of the cascades in a gas centrifuge plant after they have handled radioactive uranium hexafluoride is an undertaking comparable in scale to interconnecting all of the gas centrifuges of the plant initially.